Quadrupole ion trap device and methods of operating a quadrupole ion trap device

ABSTRACT

A quadrupole ion trap device has a field adjusting electrode located outside the trapping region adjacent the aperture in the entrance end cap electrode, and optionally adjacent the aperture in the exit end cap electrode. The field adjusting electrode(s) controls field distortion in the vicinity of the apertures. By appropriately setting the voltages on the field adjusting electrodes the efficiency and resolution of operational processes such as ion introduction, precursor ion isolation and mass scanning can be improved.

RELATED APPLICATIONS

This application is the National Stage of International Application No.PCT/GB02/04807, filed Oct. 24, 2002, entitled A QUADRUPOLE ION TRAPDEVICE AND METHODS OF OPERATING A QUADRUPOLE ION TRAP DEVICE, whichclaims priority benefit of United Kingdom (GB) Application No.0126525.5, filed Nov. 5, 2001. This U.S. National Stage Application alsoclaims priority benefit of United Kingdom (GB) Application No.0126525.5, filed Nov. 5, 2001.

FIELD OF THE INVENTION

This invention relates to quadrupole mass spectrometry. In particular,the invention relates to a quadrupole ion trap device and methods ofoperating a quadrupole ion trap device.

BACKGROUND OF THE INVENTION

The quadrupole ion trap has been developed and, used in practice, as amass spectrometer since the mass selective instability mode was inventedseveral decades ago. This technique is described in U.S. Pat. No.4,540,884. Later, in a series of the US patents such as those numberedU.S. Pat. Nos. 4,736,101, 4,749,860, 4,882,484, methods of MS and MS/MSemploying resonance ejection of ions from the ion trap have beendisclosed. Based on these methods, commercial ion trap mass spectrometerinstruments have been manufactured and widely used The principle ofoperation of these instruments can be summarised by the followingoperational steps: Simultaneously trap the ions with a predefined massrange by applying a trapping RF voltage to the ion trap; applysupplementary AC voltage between the two end cap electrodes to causeresonance ejection of unwanted ions and again, use the supplementary ACvoltage to activiate the remaining precursor ions to cause theircollisional dissociation and produce product ions; and finally, scan oneparameter of the trapping RF voltage or supplementary AC voltage tocause resonance ejection of ions sequentially in the order of theirmass-to-charge ratios. Thus, by measuring the ejected ion current a massspectrum can be obtained.

As the technology has developed, performance has been improved by addinghigh order multipole electric field components, in particular theoctapole component to the quadrupole electric field. Technically, thiswas achieved by stretching the distance between the end caps of the iontrap or by decreasing the asymtotic cone angle of the hyperboloidgeometry. These are permanent, structural changes that give rise tonon-linear resonance of ion motion; so, these ion traps are also callednon-linear ion traps. However, while the non-linear resonance which iscaused by the high order multipole components brings about certainimprovements in performance, such as good mass resolution at fast scan,it also introduces many problems. A quadrupole ion trap with significanthigh order multipole components cannot work in the mass-selectivestorage mode as is usual in the case of a quadrupole mass filter,because the non-linear resonance line which runs through the apex regionof the well known (a-q) stability diagram causes ion loss. Furthermore,the non-linear ion trap cannot provide high resolution for precursor ionselection when the resonance ejection method is used.

U.S. Pat. No. 5,468,958 (Franzen and Wang) discloses a method fordividing each end cap electrode into component parts to allow the highorder multipole part of the field to be selectively switched on or off.It is claimed that this kind of ion trap is able to store ionsselectively with good resolution, and scan out the stored ions with goodresolution as well. In practice, however, there is no easy way toimplement such a device because both RF switching and precise tuning ofcoupling parameters are difficult to achieve. Also, no account is takenof the problem of field distortion near the end cap apertures.

Recent studies by G. Cooks published in Analytical Chemistry Vol. 72 No.13, 2667, demonstrates that the end cap apertures where ions enter andexit the ion trap are the principal source of distortion in thequadrupole field. Such distortion causes chemical shift and delayedejection which leads to poor resolution of mass analysis. Adding in ahigh order multipole field, as is done in some commercial instruments,can avoid the adverse effects of the aperture, giving improvedanalytical performance, but at the same time, introduces theafore-mentioned problems associated with high order multipole fields.The present inventors have discovered that by reducing field distortionin the vicinity of the aperture of an end cap electrode high massresolution can be achieved without a significant high order multipolefield. With an adjustable small high order field near the aperture therecould be the opportunity to obtain even better results.

It is an object of the present invention to at least alleviate theafore-mentioned problems.

SUMMARY OF THE INVENTION

According to one aspect of this invention, there is provided aquadrupole ion trap device comprising an electrode structure having aring electrode, and two end cap electrodes enclosing a trapping region,one said end cap electrode being an entrance end cap electrode having acentral aperture through which ions can enter the trapping region, afield adjusting electrode located outside the trapping region adjacentto the aperture of said entrance end cap electrode, AC power supplymeans arranged to supply AC voltage to said electrode structure tocreate within the trapping region a trapping electric field for trappingions and an excitation electric field for resonantly exciting ionstrapped by the trapping electric field, and DC power supply meansarranged to supply to said field adjusting electrode, and controllablyvary, DC voltage whereby selectively to influence ion motion in thetrapping region according to an operating mode of the ion trap device.

According to another aspect of the invention, there is provided a methodof operating a quadrupole ion trap device including a ring electrode,and two end cap electrodes enclosing a trapping region one said end capelectrode being an entrance end cap electrode having a central aperturethrough which ions can enter the trapping region, and a field adjustingelectrode located outside the trapping region adjacent to the apertureof said entrance end cap electrode, the method including, generating atrapping electric field within the trapping region, generating anexcitation electric field within the trapping region for resonantlyexciting ions trapped by the trapping electric field, applying DCvoltage to said field adjusting electrode to influence ion motion nearthe entrance aperture, and selectively controlling the applied DCvoltage to improve efficiency with which ions enter the trapping regionthrough said entrance aperture and to enhance resolution of massisolation carried out on the trapped ions.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are now described, by way of example only,with reference to the accompanying drawings, of which:

FIG. 1 is a block diagram showing a quadrupole ion trap device accordingto the invention,

FIG. 2( a) shows an embodiment of a quadrupole ion trap device accordingto the invention having two field adjusting electrodes, one locatedadjacent to the entrance aperture and another located adjacent to theexit aperture,

FIG. 2( b) shows another embodiment of a quadrupole ion trap deviceaccording to the invention having a single field adjusting electrodelocated adjacent the entrance aperture and a metal mesh covering theexit aperture,

FIG. 3 shows the (a-q) stability diagram obtained for ion motion in aquadrupole electric field produced by a square waveform drive voltageand demonstrates the effect of the field adjusting electrode on secularfrequency of ions as they approach a resonance line,

FIGS. 4( a) and 4(b) respectively illustrate the variation of amplitudeof ion oscillation as a function time during a scanned resonanceejection (at resonance line β_(z)=0.5) obtained using a commercial‘stretched’ ion trap device and an ion trap device according to theinvention having a field adjusting electrode adjacent the entranceaperture,

FIGS. 5( a) and 5(b) show ranges of mass-to-charge ratio of ions ejectedfrom the trapping region using a single frequency excitation field whenthe DC voltage applied to the field adjusting electrode is 120V and 1.5kV respectively,

FIGS. 6( a) and 6(b) show a variation of ion ejection probability as afunction of mass-to-charge ratio m/z obtained using respective clippingprocesses in a notched broad band precursor ion isolation methodaccording to an aspect of the invention.

FIG. 7 is an example of an operating program for a tandem MS showing howrectangular waveform frequency, field adjusting voltage and excitationvoltage vary as a function of time during ion introduction, precursorion isolation and mass scanning processes, and

FIG. 8 is a simulation showing how efficiency of ion introduction variesas a function of voltage V_(fa) applied to the field adjustingelectrode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, the quadrupole ion trap device comprises a ringelectrode 1, an entrance end cap electrode 2 having a central entranceaperture, and an exit end cap electrode 3 having a central exitaperture, and these components enclose the trapping region R of the iontrap device.

The device also includes a pair of field adjusting electrodes 4, locatedoutside the trapping region. One of the field adjusting electrodes 4 islocated adjacent to the entrance aperture of the entrance end capelectrode 2 and another field adjusting electrode 4 is located adjacentto the exit aperture of the exit end cap electrode 3, although thisfield adjusting electrode could optionally be omitted, as will bedescribed later.

Ions produced in an ion source 9 are guided and focussed by conventionalion optics and are introduced into the trapping region R through anaperture in the field adjusting electrode 4 and then through theentrance aperture in the entrance end cap electrode 2. Ions exit thetrapping region R through the exit aperture in the exit end capelectrode 3 and then through an aperture in the associated fieldadjusting electrode 4 (if present), and are detected by a detector 8.

A voltage source 5 supplies AC trapping voltage to the ring electrode 1to generate a trapping electric field in the trapping region R. Thetrapping voltage may be a sinusoidal RF voltage with an optional DCcomponent, but is preferably a rectangular waveform trapping voltage. Ina preferred implementation, the rectangular waveform trapping voltage isgenerated digitally by controllably switching between high and lowvoltage levels to control the frequency and waveshape of the rectangularwaveform voltage. Such control enables a parameter of the trappingvoltage (e.g. frequency, amplitude) to be varied to facilitate certainoperational functions, such as precursor ion isolation and massscanning, as will be described later. The principle of such control hasbeen disclosed in WO 01/29875.

A voltage source 6 supplies an AC excitation voltage to the end capelectrodes 2,3. The AC excitation voltage is used to create a dipole orquadrupole excitation electric field in the trapping region.

DC voltage sources 7 supply DC voltage to the field adjusting electrodes4. As will be explained, the voltages supplied to the field adjustingelectrodes are controllably adjustable to facilitate differentoperational modes of the device. In one implementation, the output of avoltage source 7 is controllably selectable from one of a number (e.g.3) of different voltage levels depending on the mode of operation.

When the voltage on the field adjusting electrode 4 is set at certainvalue, negative for positive ions and positive for negative ions, ionsin a certain range of mass-to-charge ratio can be simultaneously trappedin the trapping region R with the assistance of buffer gas. Ions can bescanned out of the trapping region by the well known technique ofresonance ejection for detection by the detector 8. Scanning can beachieved by either ramping up the trapping voltage or by progressivelyreducing the frequency of the RF power supply or rectangular wavedriver. Axial excitation for ion ejection can be achieved by dipoleexcitation and/or quadrupole excitation, both being well known priorart.

During resonance ejection (for a forward scanning process), the secularfrequency ω_(z) of an ion of given mass-to-charge ratio (m/z) approachesalong the q axis the excitation frequency ω_(o) corresponding to aresonance line in the (a-q) stability diagram having a value β_(z),given by the expression

$\beta_{z} = \frac{2\omega_{o}}{\Omega}$where Ω is the angular frequency of the RF trapping voltage. As theamplitude of ion motion grows, ions start to feel the effect of thenegative high order multipole field due to the aperture in the end capelectrode. The secular frequency of the ion is reduced and an ion whichis supposed to be ejected loses its phase matching with the excitationfield, and thus the oscillation amplitude decreases. The ejectionprocess is therefore prolonged and poor mass resolution and chemicalshift result.

However, the reduction of secular frequency near the aperture is nowavoided by applying a certain DC voltage to the field adjustingelectrode 4. It is even possible by applying extra DC voltage toincrease the secular frequency causing phase matching with theexcitation field, and so ejection of the ion, to occur faster.

Referring to the FIG. 2, two embodiments of field adjusting electrodesare shown. Whilst embodiment (a) employs two field adjusting electrodes4, one behind each end cap electrode 2 and 3, embodiment (b) employsonly one field adjusting electrode 4 behind the entrance aperture, and afine mesh 10 covers the exit aperture. Both embodiments use electrodegeometries that generate pure quadrupole electric field in the trappingregion.

Now, a detailed example is given for analysing the resonance ejectionprocess. Here the ion has a positive charge and the driving voltage is+/−1 kV and has a rectangular waveform which can be scanned by varyingthe trapping frequency Ω=2πf, where f is the repetition rate of thewaveform. Axial excitation is accomplished by applying a dipolerectangular wave voltage, generated by AC excitation source 6, betweenthe two end cap electrodes 2, 3. For resonance at relatively large β_(z)values (say β_(z)>0.4) an ion approaches the end cap apertures onlyduring the negative phase of the trapping field at which time the ringelectrode is charged at −1 kV. Here, β_(z) is the value of a resonanceline in the (a-q) stability diagram given by

${\beta_{z} = \frac{2\omega_{o}}{\Omega}},$where ω_(o) is excitation frequency of the excitation electric fieldwhich, at resonance, is the same as the axial secular frequency ω_(z).In FIG. 2, when the field adjusting electrode(s) are supplied with 1.5kV DC, the equipotential surfaces 11 do not show much field distortionnear the apertures of the end cap electrodes 2,3. In this case, an ioncan maintain its secular oscillation frequency until it hits an end capelectrode or exits the trapping region through one of the apertures. Infact, a simulation of the ion motion shows that mild acceleration of theejection process occurs during a forward mass scan (i.e. a scan in whichions are ejected from the trapping region sequentially in the order ofincreasing mass-to-charge ratio), accomplished by progressively reducingthe trapping frequency, for example.

This can be explained by reference to FIG. 3 which shows how the workingpoint W in the (a-q) stability diagram of an ion of given mass-to-chargeratio moves along line a=0 towards a resonance line (in this caseβ_(z)=0.5) as the forward mass scan progresses. As the ion approaches anend cap electrode 2,3 it sees an average DC field created by the voltageapplied to the associated field adjusting electrode 4. This DC offsetcauses an up-shift of the working point to a finite value of theparameter a, closer to the resonance line, thereby abruptly driving theion into the resonance condition and speeding up its ejection.

FIGS. 4 a and 4 b show simulations of the amplitude of axial excursionsof the ions as a function of time as the ions undergo resonanceejection, scan in a stretched geometry ion trap device (FIG. 4 a) and inthe ion trap shown in FIG. 2 b (FIG. 4 b). Each illustration shows theamplitude of axial excursions of two ions having the same mass-to-chargeratio (1750 Th) which are randomised by collisions with buffer gas.

FIG. 4 a show that a strong beat is present in the trajectories producedin the stretched geometry ion trap, and the ejection times will dependupon the phase of this beat which is, of course, a random factor. Growthof the axial excursions of the trajectories shown in FIG. 4 b issteadier, and the ejection times for the two ions are much closer,although acceleration towards the resonance condition is not aspronounced.

In the case of the ion trap device shown in FIG. 2( b), application of apositive DC voltage to the field adjusting electrode adjacent theentrance end cap electrode 2 causes all positive ions to be ejectedthrough the mesh covered aperture in the exit end cap electrode 3 fordetection, and this increases the sensitivity of the measurement.

As explained, a high positive voltage applied to the field adjustingelectrode(s) enhances the performance of a forward mass scan, in whichthe axial secular frequency ω_(z) of ions is matched to the excitationfrequency ω_(o) by shifting the working point of the ions from left toright in FIG. 3 until a resonance line is reached. However, applicationof a much smaller DC voltage (say, 120 V for example) to the fieldadjusting electrode(s) 4 can accelerate the ejection of ions during areverse mass scan (i.e. a scan in which ions are ejected sequentially inorder of decreasing mass-to-charge ratio) when the axial secularfrequency ω_(z) of ions is matched to the excitation frequency ω_(o) ofthe excitation voltage by shifting the working point from right to leftin FIG. 3 until a resonance line is reached.

It is impossible in a commercial ion trap device using a positiveoctapole field to improve mass resolution during forward mass scan, alsoto achieve high mass resolution during a reverse scan.

However, in the case of the present invention, most of the trappingregion is situated in a pure quadrupole electric field in which thetrajectories of the ions' oscillations can steadily expand during aresonance ejection scan. During a reverse mass scan, ions will approachthe resonance line from the right hand side of the a-q stabilitydiagram, or, in other words, the secular frequency of the ions decreasesuntil it matches the excitation frequency ω_(o). The ions see thenegative high order multipole field at the aperture because the positivecompensating field produced by the field adjusting electrode 4 isrelatively small. This negative high order field leads to a decrease ofsecular frequency, abruptly driving the ion towards the resonancecondition and speeding up its ejection.

Above, we have shown that good mass scan performance can be achieved bycontrolling the voltage applied to the field adjusting electrode(s). Wenow present some simulation results showing ejection probability atgiven resonant ejection conditions, and we discuss different methods forprecursor ion isolation.

For a given trapping field, a single excitation frequency should,according to theory, cause ejection of ions having a singlemass-to-charge ratio. However, in practice, there is a finiteprobability that ions having higher and lower mass-to-charge ratios willalso be ejected, reducing the mass resolution of the resonance ejectionprocess. However, application of high and low DC voltages to the fieldadjusting electrode(s) can significantly improve the mass resolution ofthis process.

FIGS. 5 a and 5 b show mass ejection bands obtained, by simulation,using a single excitation frequency and fixed trapping parameters. Inthese simulations ten ions were used for each mass-to-charge ratio andall ions were excited using the same dipole excitation field. Thesefigures respectively illustrate the effect of applying a low DC voltage(V_(fa)=120V) and a high DC voltage (V_(fa)=1.5V) to the field adjustingelectrode(s).

The effect of applying the low DC voltage to the field adjustingelectrode(s) is to create a steep clipping edge on the low mass side ofthe resultant ejection band, whereas the effect of applying the high DCvoltage to the field adjusting electrode(s) is to create a steepclipping edge on the high mass side of the resultant ejection band.

These steep clipping edges can be exploited to improve resolution ofprecursor ion isolation, and this simulation shows that it is possibleto isolate precursor ions having a single mass-to-charge ratio (3500 inthis example), as will be explained.

To this end, the afore-mentioned forward and reverse mass scans can becombined to isolate, with high resolution, precursor ions having asingle (or small range of) mass-to-charge ratio. In this application,the forward mass scan is carried out to eject ions having mass-to-chargeratios smaller than that of the selected precursor ions, and the reversemass scan is then carried out to eject ions having mass-to-charge ratioslarger than that of the selected precursor ions. Both scans would stopjust short of the mass-to-charge ratio of the selected precursor ions.The order of the two scans can be interchanged, but for each scan thevoltage (V_(fa)) on the field adjusting electrodes is set at theappropriate value (i.e. a high value for the forward mass scan and amuch smaller value for the reverse mass scan) in order to create theafore-mentioned steep clipping edges on the high and low mass sidesrespectively of their respective mass ejection bands. This processenables the mass-to-charge ratio of the isolated precursor ions to bedefined with high mass resolution.

Between the forward and reverse mass scans the ions remaining in thetrapping region are subjected to a cooling process.

It is also very common to use a notched broad band excitation signal,applied between the end cap electrodes, to excite ions to effectprecursor ion isolation.

For a fixed trapping field, ions having mass-to-charge ratioscorresponding to the excitation frequencies of the broadband excitationfield will be resonantly excited and thereby ejected from the trappingregion including those hitting the end cap electrodes.

In the notch, where excitation frequencies are absent, ions havingcorresponding mass-to-charge ratios will be retained.

The notch in the excitation signal is defined by upper and lowerfrequency limits, respectively corresponding to lower and upper masslimits of a range of mass-to-charge ratio. The current invention offersthe possibility to sharply cut away unwanted ions from both the low andthe high mass sides of this mass range. To this end, V_(fa) should beset at a value such that the secular frequency shift which occurs asions approach the apertures of the electrodes is minimised. In thisexample, V_(fa)=1.3 kV, giving good conditions for ejection of unwantedions on both sides of the precursor ion mass range that is to beisolated. However, a two stage clipping method is expected to give evenbetter resolution.

The frequency notch in the frequency spectrum of the notched broadbandexcitation signal corresponds to a range of a mass-to-charge ratio.

In a first stage of the two stage clipping method, V_(fa) is set at120V, creating a sharp clipping edge on the high mass side of the massrange, and so defining an upper mass limit. The selected mass-to-chargeratio is set just below the upper mass limit so that substantially allions having mass-to-charge ratios greater than the selectedmass-to-charge ratio are ejected from the trapping region. This is theequivalent to setting the secular frequency of the precursor ions justabove the lower frequency limit of the frequency notch.

In a second stage of the clipping method, V_(fa) is set at 1.5 kVcreating a sharp clipping edge on the low mass side of the mass range,and so defining a lower mass limit. The selected mass to charge ratio isset just above the lower mass limit so that substantially all ionshaving mass-to-charge ratios less than the selected mass-to-charge ratioare ejected from the trapping region. This is equivalent to setting thesecular frequency of the precursor ions just below the upper frequencylimit of the frequency notch.

The mass range of ions remaining within the trapping region at theconclusion of the two stage clipping process will be determined by thecloseness of the selected mass-to-charge ratio to the upper and lowermass limits in the two clipping stages, but not the width of the notch.The described process enables precursor ions having a singlemass-to-charge ratio to be isolated.

FIGS. 6 a and 6 b illustrate ejection probability as a function ofmass-to-charge ratio m/z obtained by the respective stages of thisclipping method.

The position of the upper and lower mass limits can be set relative tothe selected mass-to-charge ratio by controllably adjusting the trappingelectric field (by adjusting the frequency and/or amplitude of the drivevoltage) or by controllably shifting the position of the frequency notchwithin the frequency spectrum of the broadband excitation signal.

The order of the first and second stages of the two stage clippingprocess can be reversed so that the low mass side of the isolated massrange is clipped before the high mass side.

Between the two stages, the ions remaining in the trapping region aresubjected to a cooling process.

FIG. 7 illustrates an example of precursor ion isolation using thetwo-stage, notched broadband frequency clipping process. This Figurealso illustrates ion introduction and mass scanning.

One more aspect of using a voltage controllable field adjustingelectrode is to improve the efficiency with which ions are introducedinto an ion trap employing an external ion source. In principle, ionsgenerated outside the quadrupole ion trap cannot be trapped if the iontrap is driven by a fixed, periodically changing AC voltage. This can beexplained, by observing that the energy of an ion which is able to enterthe trapping region must be higher than the depth of the pseudopotential well and so it must have a high kinetic energy, i.e. enoughenergy to escape from the ion trap or to hit an internal surface of theion trap. Damping gas, normally helium or nitrogen, is used to removethe kinetic energy of the injected ions by collisons. This improves thechances that the ions will be trapped. However removal of sufficientenergy within one secular swing, so that ions will not collide with thesurface of an electrode is less probable. So the trapping efficiency isnormally very low.

Now, by using a field adjusting electrode at the entrance aperture andapplying it to a negative voltage, the potential well during a certainphase of the trapping field may be modified to look like a well with anarrow notch on its edge. An ion entering the well may carrysubstantially the same or less energy than the depth of the well and ittakes a relatively long time for the ion to find the notch again andescape, so there is greater probability that the kinetic energy of theion will have been reduced and that the ion will be permanently trapped.Because V_(fa) is adjustable, it can be tuned to trap ions withdifferent initial parameters such as mass-to-charge ratio and energyduring the introduction period.

FIG. 8 shows the trapping efficiency, obtained by simulation, during ionintroduction. In this simulation, ion mass was 6000 Da, the initialkinetic energy of the ions was 15 eV staring from a lens system held atan electrical potential of −20V. The ion beam had a Gaussian radialdistribution, with σ=0.1 mm. The ions underwent random collision with Hebuffer gas and the mean free path was assumed to be 5 mm. If the massrange of trapping is not a priority, it is suggested to use a certain DCcomponent in the trapping field (a≠0). In such cases, the radial secularfrequency will differ from the half frequency of the axial secularoscillations, so it is harder for ion to return to the entranceaperture.

The voltage on the field adjusting electrode(s) can be supplied by avoltage controllable DC power supply. The means to control the voltagecan be either switching means or a linear control means such as afeedback loop. The output should have at least three selectivelyswitchable voltage levels to accommodate introduction, ion isolation(which requires two levels) and mass scanning.

The field adjusting electrode 4 should be placed close enough to an endcap aperture (a distance from the aperture less than or equal to thediameter of the aperture) to ensure that the electrode has a sufficientinfluence in the aperture region inside the trap. Although in theillustrated embodiments the electrode has a solid structure with anaperture aligned with the entrance aperture of the end cap electrode, itcan also be formed as a metal grid or may be made of solid metal butwith a mesh covering its aperture.

When two field adjusting electrodes are used to compensate for the fielddistortion due to both end cap electrode apertures, as shown in FIG. 2a, mass resolution for precursor ion selection can be improved withoutthe complication of a mesh structure. However, ions may not besuccessfully ejected through the end cap aperture because the requiredvoltage on the field adjusting electrode for multiple field correctionalways retards the ions. Therefore, the structure of FIG. 2 a is notsuitable for mass analysis in the resonant ejection scan mode, but itmay be favourable when the ion trap is used as an ion selection means intechnology such as an ion-trap-ToF tandem MS application. It can also beused for mass analysis by detecting image current induced by the secularmotion of ions.

The foregoing embodiments have been described with reference topositively charged ions. In the case of negatively charged ions, fieldadjusting electrode(s) would be supplied with DC voltages having theopposite polarities.

The invention also relates to a mass spectrometer comprising thecombination of an ion source, such as an electrospray ion source havingthe necessary high pressure-to-vacuum interface, an ion trap device, inaccordance with the invention, as described in any of the foregoingembodiments and ion optics to guide and focus ions from the ion sourceinto the ion trap device. To detect ions ejected from the ion trapdevice a detector in the form of a conventional electron multiplierhaving a conversion dynode can be used. Alternatively, a multi-channelplate (MCP) or a cryogenic detector for ions of very high mass could beused.

A mass spectrometer may use the field-adjustable ion trap device as astore and precursor ion selection tool, and may include a ToF to achievefast and accurate mass analysis. In this case, ions are firstlyintroduced with high efficiency to the ion trap device where highresolution precursor selection can be carried out. The isolatedprecursor ions can then be excited and made to collide with neutral gasmolecules or with an ion trap electrode to cause dissociation (CID andSID) of the precursor ions. The resultant product ions are finallyejected into the ToF analyser by applying pulsed voltage between the twoend cap electrodes. Because the final mass validation is obtained byusing ToF, control of the voltage of field adjusting electrode to keephigh mass scan resolution is not used. Instead, when using pulsedejection, the voltage on the field adjusting electrode near the exit endcap electrode should be set at a potential for making ejection easierand for enabling a better ion beam to be formed for introduction intothe ToF. In this case it is preferable to use, a negative voltage forejection of positive ions and a positive voltage for the ejection ofnegative ions.

1. A quadrupole ion trap device comprising, an electrode structurehaving a ring electrode and two end cap electrodes enclosing a trappingregion, one said end cap electrode being an entrance end cap electrodehaving a central aperture through which ions can enter the trappingregion, a field adjusting electrode located outside the trapping regionadjacent to the aperture of said entrance end cap electrode, AC powersupply means arranged to supply AC voltage to said electrode structureto create within the trapping region a trapping electric field fortrapping ions and an excitation electric field for resonantly excitingions trapped by the trapping electric field, and DC power supply meansarranged to supply to said field adjusting electrode, and controllablyvary, DC voltage whereby selectively to influence ion motion in thetrapping region according to an operating mode of the ion trap device.2. A device as claim 1 including a further field adjusting electrodelocated outside the trapping region adjacent to the aperture of anothersaid end cap electrode being an exit end cap electrode, and wherein saidDC power supply means is arranged to supply DC voltage to said furtherfield adjusting electrode and to controllably vary the supplied voltageto influence ion motion near the aperture of said exit end capelectrode.
 3. A device as claimed in claim 1 wherein the aperture ofanother said end cap electrode being an exit end cap electrode isadapted to minimise influence of that aperture on the shape ofequipotential field surface inside the trapping region.
 4. A device asclaim 3 wherein the aperture of said exit end cap electrode has an iontransmissive, electrically conductive covering.
 5. A device as claim 4wherein said covering is a metal mesh.
 6. A device as claim 3 whereinthe aperture of the exit end cap electrode is smaller than the apertureof the entrance end cap electrode.
 7. A device as claimed in claim 1wherein said DC power supply means supplies to said field adjustingelectrode DC voltage controllably selectable from a plurality ofdifferent voltage levels according to the operational mode of thedevice.
 8. A device as claim 7 wherein said DC voltage is controllablyselectable from three said voltage levels, a first said voltage levelbeing selected while ions are being introduced into the trapping region,a second said voltage level being selected while ions are being ejectedfrom the trapping region, for analysis, during a mass scanning mode ofoperation, and said second and third said voltage levels being selectedduring a precursor ion isolation mode of operation.
 9. A device asclaimed in claim 1 wherein said ring electrode and said end capelectrodes have a hyperboloid geometry.
 10. A device as claimed in claim1 wherein said AC power supply means includes a RF voltage source forsupplying drive voltage to the ring electrode wherein the frequencyand/or amplitude of the drive voltage supplied to the ring electrode canbe scanned across a predetermined range to reasonably excite, and ejectfrom the trapping region, ions selected sequentially in the order oftheir mass-to-charge ratios.
 11. A device as claimed in claim 1 whereinsaid AC power supply means includes switching means for supplying arectangular waveform drive voltage to the ring electrode wherein aparameter defining said rectangular waveform drive voltage can bescanned across a predetermined range to resonantly excite, and ejectfrom the trapping region, ions selected sequentially in the order oftheir mass-to-charge ratios.
 12. A device as claim 11 wherein saidswitching means is a digitally controllable switching means.
 13. Adevice as claimed in claim 1 wherein said DC power supply means isarranged to scale said DC voltage in proportion to the trapping voltagesupplied to the ring electrode.
 14. A method for using an ion trappingdevice as claimed in claim 1 to isolate precusor ions having a selectedmass-to-charge ratio, the method comprising the steps of performing twomass scanning procedures, one said mass scanning procedure beingeffective to resonantly excite, and thereby remove from the trappingregion, ions sequentially in the order of increasing mass-to-chargeratio up to and including a mass-to-charge ratio less than said selectedmass-to-charge ratio, and another said mass scanning procedure beingeffective to resonantly excite, and thereby remove from the trappingregion, ions sequentially in the order of decreasing mass-to-chargeratio down to and including a mass-to-charge ratio greater than saidselected mass-to-charge ratio, setting the DC voltage supplied to saidfield adjusting electrode at a first voltage level while said one massscanning procedure is being carried out and setting the DC voltage at asecond voltage level, having a magnitude less than that of said firstvoltage level, while said another mass scanning procedure is beingcarried out, and cooling ions that remain in the trapping region betweenperformance of said one and another mass scanning procedures.
 15. Amethod as claimed in claim 14 wherein said AC power supply meanssupplies a rectangular waveform drive voltage to said ring electrode tocreate said trapping electric field, and said one and another massscanning procedures are carried out by scanning a parameter of therectangular waveform drive voltage across different respective ranges.16. A method of using an ion trapping device as claimed in claim 1 toisolate precursor ions having a selected mass-to-charge ratio, themethod including, creating a notched broadband excitation electric fieldhaving a frequency notch corresponding to a range of mass-to-chargeratio, performing a two-stage clipping method, one said stage of theclipping method including setting the voltage applied to said fieldadjusting electrode at a first voltage level to create a clipping edgeon the low mass side of said mass range defining a lower mass limit andsetting said selected mass-to-charge ratio close to said low mass limit,and another said stage of the clipping method including setting thevoltage applied to said field adjusting electrode at a second voltagelevel, having a magnitude less than said first voltage level, to createa clipping edge on the high mass side of said mass range defining anupper mass limit and setting said selected mass-to-charge ratio close tosaid upper mass limit, and cooling ions that remain in the trappingregion between performance of the two clipping method.
 17. A method asclaimed in claim 16 wherein said one said stage of the clipping methodis effective to eject substantially all ions having mass-to-chargeratios less than said selected mass-to-charge ratio and said anothersaid stage of the clipping method is effective to eject substantiallyall ions having mass-to-charge ratios greater than said selectedmass-to-charge ratio so that at the conclusion of said one and anotherclipping methods the only ions remaining with the trapping region areions having said selected mass-to-charge ratio.
 18. A method as claimedin claim 16 wherein a position of said selected mass-to-charge ratiorelative to said upper and lower mass limits is set by controllablyadjusting the trapping electric field.
 19. A method as claimed in claim16 wherein a position of said selected mass-to-charge ratio relative tosaid upper and lower limits is set by controllably shifting the positionof said frequency notch whereby to shift said range of mass-to-chargeratio relative to said selected mass-to-charge ratio.
 20. A method forusing an ion trapping device as claim 1 to isolate precursor ions havinga selected mass-to-charge ratio, the method including: creating anotched broadband excitation electric field having a frequency notchdefined by upper and lower frequency limits, performing two massclipping processes, one said mass clipping process including setting theDC voltage applied to said field adjusting electrode at a first voltagelevel and setting the secular frequency of the precursor ions closer tothe upper frequency limit than the lower frequency limit, and anothersaid mass clipping process including setting the DC voltage applied tosaid field adjusting electrode at a second voltage level; having amagnitude less than that of said first voltage level and setting thesecular frequency of the precursor ions closer to the lower frequencylimit than the upper frequency limit, and cooling the ions that remainin the trapping region between performance of the two mass clippingprocesses.
 21. A mass spectrometer comprising an ion source, aquadrupole ion trap device as claimed in claim 1, ion optics for guidingand focussing ions from the ion source into the ion trap device, andmeans for detecting ions ejected from the ion trap device.
 22. A massspectrometer comprising an ion source, a quadrupole ion trap device asclaimed in claim 1, ion optics for guiding and focussing ions from theion source into the ion trap device and time-of-flight means foranalysing ions ejected from the ion trap device.
 23. A method ofoperating a quadrupole ion trap device including a ring electrode, andtwo end cap electrodes enclosing a trapping region, one of said end capelectrodes being an entrance end cap electrode having a central aperturethrough which ions can enter the trapping region, and a field adjustingelectrode located outside the trapping region adjacent to the apertureof said entrance end cap electrode, the method including, generating atrapping electric field within the trapping region, generating anexcitation electric field within the trapping region for resonantlyexciting ions trapped by the trapping electric field, applying DCvoltage to said field adjusting electrode to influence ion motion nearthe entrance aperture, and selectively controlling the applied DCvoltage to improve efficiency with which ions enter the trapping regionthrough said entrance aperture and to enhance resolution of massisolation carried out on the trapped ions.
 24. A method as claimed inclaim 23 including selectively controlling the applied DC voltage toenhance resolution of a mass-selective scanning process carried out onthe trapped ions.
 25. A method as claimed in claim 24 wherein saidmass-selective scanning process includes precursor ion selection and/orejection from the trapping region, for analysis, of ions sequentially inthe order of their mass-to-charge ratios.
 26. A method as claimed in anyone of claims 23 to 25 wherein the applied DC voltage compensates for areduction of ion secular frequency caused by high order multipole fieldsnear the entrance end cap electrode.
 27. A method as claimed in any oneof claims 23 to 25 wherein the applied DC voltage causes an increase ofion secular frequency as the axial excusions of the trajectories of theions approach the entrance aperture within the trapping region.
 28. Amethod as claimed in any one of claims 23 to 25 wherein said trappingelectric field is generated by supplying RE voltage to said ringelectrode, and said DC voltage is scaled to be in proportion to theamplitude of the RF voltage during a said mass-selective scanningprocess.
 29. A method as claimed in claim 23 wherein the DC voltageapplied to said field adjusting electrode is set to have a polarityopposite to that of the ions to be trapped and at such a level as toassist entry of the ions into the trapping region through the apertureof the entrance end cap.
 30. A method as claimed in claim 29 includingproviding a DC component in the trapping electric field to inhibit ionsintroduced into the trapping region from immediately returning to theentrance aperture.